The invention relates to a dynamic focusing circuit comprising a transformer with a primary winding for receiving a line-frequency deflection current and a secondary winding for supplying a transformed deflection current, and an integrator, coupled to the secondary winding, for generating a line-frequency focusing voltage from the transformed deflection current.
The invention also relates to a picture display device comprising a cathode ray tube with an electron gun provided with a DAF section, deflection means, a deflection circuit and a dynamic focusing circuit of the type described in the opening paragraph.
The invention also relates to a method of generating a line-frequency focusing voltage in a picture display device, the method comprising the steps of transforming a line-frequency deflection current to a transformed deflection current, and integrating the transformed deflection current to a line-frequency focusing voltage.
A prior-art dynamic focusing circuit generating a dynamic focusing voltage in accordance with a method described in the opening paragraph is known from, for example, U.S. Pat. No. 5,430,358.
An electron beam is imaged on a display screen of a picture display device comprising a cathode ray tube. The electron beam is generated by the electron gun in the cathode ray tube and imaged on the display screen. The display screen is provided with rows or dots of phosphors which luminesce when they are impinged upon by the electron beam.
The picture display device comprises deflection means for deflecting an electron beam in the cathode ray tube. The deflection means are controlled by a deflection circuit in the picture display device. This deflection circuit generates a line-frequency deflection current and a field-frequency deflection current, synchronized with a video signal received by the picture display device. In this way, the landing spot of the electron beam on the display screen changes and the video signal is displayed on the display screen.
By deflecting the electron beam by way of the deflection means, the electrons between the electron source and the landing spot on the display screen travel a longer path. More particularly, the electrons between a main lens, which focuses the electron beam, and the display screen travel a longer path in dependence upon the deflection. As a result, the electron beam is no longer substantially focused throughout the display screen, and the electron beam is imaged relatively out of focus on at least a part of the display screen.
When deflecting the electrons, the deflection means also act as an electron-optical quadrupolar lens so that astigmatism occurs and the shape of the electron beam changes in dependence upon the deflection.
These effects are most pronounced in the corners of the display screen, i.e. when the deflection of the electrons is greatest in both the line-frequency direction and the field-frequency direction.
To reduce these effects, the electron gun may be provided with a section referred to as DAF section as is known from, for example, U.S. Pat. No. 4,742,279. The DAF section comprises a further electron-optical quadrupolar lens whose power can be dynamically varied in dependence upon the deflection of the electrons so as to at least partly correct astigmatism caused by the deflection means. Moreover, the DAF section may change the power of the main lens so that the electron beam can be substantially in focus throughout the display screen.
The DAF section receives a fixed focusing voltage and a dynamic focusing voltage supplied by a dynamic focusing circuit.
In the prior-art dynamic focusing circuit, a transformer receives the line-frequency deflection current from the deflection circuit, which deflection current is substantially sawtooth-shaped. The transformer transforms the line-frequency deflection current, whereafter the transformed line-frequency deflection current is integrated by a capacitor to a line-frequency deflection voltage. The dynamic focusing circuit also generates a field-frequency deflection voltage. In the known dynamic focusing circuit, the field-frequency deflection voltage is formed from the line-frequency deflection voltage. The line-frequency focusing voltage and the field-frequency focusing voltage are combined to a dynamic focusing voltage which is combined with a fixed focusing voltage, for example, via a coupling capacitor. The resultant voltage is applied to the DAF section of the electron gun.
It is desirable to have a relatively small depth of the cathode ray tube for a display screen having a relatively large surface area. This means that the electron beam is deflected through relatively large angles, for example, 120xc2x0. At such deflection angles, the electron beam near the edges of the display screen, and particularly in the corners of the display screen, is considerably out of focus and astigmatic to a relatively strong extent. The correction of these effects by the DAF section requires a dynamic focusing voltage with a relatively large amplitude, for example, 2000 volts.
Generally, the display screen has a larger dimension in the line-frequency direction than in the field-frequency direction, the ratio being, for example, 4:3 or 16:9. It is then advantageous if the line-frequency focusing voltage constitutes a considerable part of the dynamic focusing voltage, i.e. the line-frequency focusing voltage has an amplitude of, for example, 1500 volts. and the field-frequency focusing voltage has an amplitude of, for example, 500 volts.
The known dynamic focusing circuit supplies a line-frequency focusing voltage having a parabola shape which is flattened near the edges of the screen. Moreover, the line-frequency deflection current in the comers of the display screen is smaller than on the line axis of the display screen, which is due to a pincushion-shaped distortion of the frame by the deflection field.
These effects give the line-frequency focusing voltage a shape which substantially deviates from the desired shape, so that the defocusing and the astigmatism of the electron beam are insufficiently reduced near the edges of the display screen and particularly in its corners.
The ideal shape of the line-frequency focusing voltage is generally a shape referred to as bathtub shape. A bathtub shape is herein understood to mean a substantially fourth-order shape which, as compared with a parabola shape, is relatively flat near a center of the display screen and relatively steep near the edges of the display screen.
In an alternative prior-art dynamic focusing circuit, the line-frequency focusing voltage is entirely generated by means of a waveform-generating circuit, while the dynamic focusing circuit does not comprise a transformer. Such a circuit is described in, for example, EP-B-0 741 948. In this circuit, the shape of the line-frequency focusing voltage can be optimally suited to the desired shape. However, this circuit has the problem that the line-frequency focusing voltage is limited to approximately 1200 volts due to transistor limitations in the waveform generator.
It is possible to use transistors which can generate a higher line-frequency focusing voltage than 1200 volts, but these transistors are expensive and have great dissipation losses.
It would be advantageous to provide a dynamic focusing circuit of the type described in the opening paragraph, which can supply, in a relatively inexpensive way, a line-frequency focusing voltage of the amplitude and shape required for a picture display device with a relatively large maximal deflection angle of the electron beam. For example, a waveform generator can be coupled to the secondary winding to generate an additional line-frequency focusing voltage and to superimpose the additional line-frequency focusing voltage on the line-frequency focusing voltage.
The circuit and the method according to the invention have the advantage that a line-frequency focusing voltage with a relatively large amplitude is generated by means of the transformer, on which focusing voltage an additional line-frequency focusing voltage is superimposed for correcting the shape of the line-frequency focusing voltage. The line-frequency focusing voltage may then have an amplitude which is larger than 1500 volts. This is particularly advantageous in a picture display device in which the electron beam is deflected through a relatively large angle.
It is advantageous if the additional line-frequency focusing voltage has a shape correcting the line-frequency focusing voltage to a bathtub shape. The line-frequency focusing voltage generally has a parabola shape which is flattened near the edges of the display screen so that it is advantageous when the additional line-frequency focusing voltage has substantially fourth-order and higher-order line-frequency components. Such a signal is capable of causing the line-frequency focusing voltage to change sufficiently fast near the edge of the display screen and can be easily generated by means of a waveform-generating circuit.
To ensure that the additional line-frequency focusing voltage satisfactorily corrects the shape of the line-frequency focusing voltage, while the total line-frequency component of the dynamic focusing voltage has a sufficiently large amplitude, it may be advantageous when the amplitude of the additional line-frequency focusing voltage and of the line-frequency focusing voltage are of the same order. The additional line-frequency focusing voltage may then have an amplitude of between 0.5 and 2 times the amplitude of the line-frequency focusing voltage.
It would also be advantageous to provide a picture display device having a relatively large maximal deflection angle, in which the electron beam is substantially in focus throughout the display screen and astigmatism of the electron beam is limited as much as possible. Accordingly, one or more embodiments of the invention include a dynamic focusing circuit that supplies a dynamic focusing voltage to the DAF section in the electron gun, which dynamic focusing voltage corresponds as much as possible to the actually required dynamic focusing voltage so that the electron beam is substantially in focus throughout the display screen and astigmatism is limited as much as possible.
It is advantageous when the primary winding of the transformer of the dynamic focusing circuit is coupled to the deflection circuit. In this way, the dynamic focusing circuit for generating the line-frequency focusing voltage can receive the line-frequency deflection current so that no extra components are required for generating said line-frequency deflection current.
It would also be advantageous to provide a method of generating , in a relatively inexpensive way, a line-frequency focusing voltage of the amplitude and shape required for a picture display device with a relatively large maximal deflection angle of the electron beam.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.